The thyroid is a butterfly-shaped gland on the neck. It produces, stores and excretes thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3). Thyroid hormones have several significant functions, and they are needed, for example, for hormone production (oestrogen) and hormone activity (growth hormone), fat, glucose and bone metabolism as well as bowel, gall bladder and liver function. In blood, 99.9% of thyroxine is bound to transport proteins and only 0.1% is unbound, i.e. free in circulation (S-T4-V). This free thyroxine is more suitable for the assessment of thyroid function, as it has hormonal functions. In the abbreviation, the number 4 refers to the fact that a thyroxine molecule contains 4 iodine molecules (Greek word thyreos means ‘shield’).
When should T4-V be measured?
Thyroid values are checked when hyperthyroidism (overactive thyroid, Greek word hyper means ‘over’ or ‘above’) or hypothyroidism (underactive thyroid, Greek word hypo means ‘under’ or ‘below’, see below) is suspected. Disorder of thyroid function can cause the following symptoms:
- Hair loss
- Intolerance to cold
- Retardation of thought
- Poor general condition
- Slow pulse
- Shortness of breath
- Muscle symptoms
- Swelling of the face and eyelids
- Weight gain
- Disturbance of the menstrual cycle
- Dry skin
- High cholesterol values (lipid values)
What does a T4-V test measure?
Testing the concentration of free thyroxine in blood (S-T4-V) provides information about thyroid function and especially the amount of functional thyroid hormone. This test is important especially if other thyroid test results are within reference values or the results are difficult to interpret.
How to interpret the T4-V result?
Normally, the result is:
- Adults: 9-19 pmol/l
The reference values of this examination have changed 11.10.2021. You will find your own result's reference values from My LOUNA in touch with the graph.
In case of hyperthyroidism (overactive thyroid) T4-V concentration is increased, and in case of hypothyroidism (underactive thyroid) the T4-V concentration is reduced. Some acute illnesses can affect the assessment of thyroid status, and it is good to check other thyroid values as well, such as thyrotropin S-TSH.
Please contact your physician or other healthcare professional if you suspect an illness or need help interpreting the results.
What can cause elevated T4-V values?
Elevated T4-V values occur in the following situations:
- Hyperthyroidism (overactive thyroid)
- After giving birth
- In connection with heparin treatment (prevents blood clots)
- In connection with treatment to restore the heart rhythm
- Illnesses related to elevated fatty acid levels
What can cause decreased T4-V values?
Typically, T4-V values are decreased due to hypothyroidism (underactive thyroid). Hypothyroidism causes extensive metabolic disorders in the body. Normally, hypothyroidism is caused by a slowly progressing autoimmune thyroiditis. In this condition, the body produces antibodies against thyroid tissues, slowly destroying the thyroid over the years. Hypothyroidism can also be caused by radioiodine therapy, i.e. oral administration of radioactive drug, or surgical removal of the thyroid gland.
Low T4-V values also occur at the end of pregnancy and in connection with some antiepileptic drugs.
The most typical T4-V additional tests:
Triiodothyronine, free, indicates hyperthyroidism (2775 S-T3-V) TSH indicates thyroid disease (2832 TSH) Thyroid peroxidase, antibodies are produced against own cells in an autoimmune disease (4028 S-TPOAb) TSH receptor antibodies (4965 TSHRAb)
Fasting is not required
This examination does not require fasting